This invention relates to circuits for electronic strobe lights utilizing microcontrollers and microprocessors. Strobe lights are used to provide visual warning of potential hazards or to draw attention to an event or activity. An important field of use for strobe lights is in electronic fire alarm systems, frequently in association with audible warning devices such as horns, to provide an additional means for alerting those persons who may be in danger. Strobe alarm circuits include a flashtube and a trigger circuit for initiating firing of the flashtube, with energy for the flash typically supplied from a capacitor connected in shunt with the flashtube. In some known systems, the flash occurs when the voltage across the flash unit (i.e., the flashtube and associated trigger circuit) exceeds the threshold value required to actuate the trigger circuit, and in others the flash is triggered by a timing circuit. After the flashtube is triggered it becomes conductive and rapidly drains the stored energy from the shunt capacitor until the voltage across the flashtube has decreased to a value at which the flashtube extinguishes and becomes non-conductive. In a more specific sense, the present invention relates to apparatus for charging the energy-storing capacitor in a more precise and efficient manner.
Underwriters Laboratories provides certain specifications that must be met by the alarms for life safety use. For example, the flash rate of the strobe must meet a minimum requirement for the range of voltages for which the flash alarm is to operate.
Supply voltage to strobe alarms, even though typically D.C., often varies in a range of 20 to 31 volts. Changes in voltage due to various conditions such as brown outs can vary the supply voltage applied to the strobe alarm during operation by as much as 4 to 5 volts. In order to ensure that the minimum energy requirements were met, strobe alarms were designed to expend the required energy for the lowest reasonably expected supply voltage. As a consequence, supply voltages greater than the lowest reasonably expected value would (1) unnecessarily expend energy in the flash above the minimum, (2) more often than needed and/or (3) in a manner that was not useful.
For example, the capacitor across the flashtube charges faster in the presence of a higher input voltage. If the flash is actuated sensing the potential across the capacitor, the frequency of the flashes increases in response to the increased input voltage. In addition to wasting energy, the increased frequency also causes unnecessary wear and tear on the capacitor and the flashtube. In another example, where the flash is actuated from a separate timing circuit, a higher input voltage will cause overcharging of the capacitor, or at least make it necessary to provide a larger capacitor than should be necessary. As a result, the potential across the capacitor will cause a larger than necessary flash, thereby wasting energy.
Whether it is the flash frequency or the flash intensity that is increased, energy is being wasted. This is of special concern when the voltage source is a battery supply. Wasted energy translates into a shorter battery life span. Thus, being able to provide precisely sufficient energy per flash at a constant frequency will permit meeting minimum standards of energy output while at the same time minimizing unnecessary expenditure of energy, number of flashes and wear and tear on all components, thus extending the life of the components.
Another problem associated with prior art strobe alarms is the surge in current caused by cycling the switch used to control the storing of the energy for the flash. By storing energy in a small duty cycle (i.e., in one flash cycle, storing energy for a number of short periods of time interspersed with longer periods of inactivity), higher peak currents are required than if longer charging periods with shorter inactive times were used. The commonly used short duty cycles increase the chances of a current overload resulting in the tripping of a circuit breaker or blowing of a fuse, especially when power from one source is supplying more than one alarm, or other devices, such as a control panel. Moreover, current surges, often maximized upon commencing charging immediately after a flash, create problems in practical applications.
In order to overcome the above-described disadvantages and shortcomings of known prior art circuits, an object of the present invention is to provide an improved strobe light circuit wherein the energy expended by the flash has decreased fluctuation, is less dependent on the supply voltage, if at all, and does not vary substantially the flash rate or the flash intensity.
Another object of the invention is to provide a strobe light circuit which provides with few components a constant flash rate and intensity.
A further object is to provide a strobe light circuit which has a higher operating efficiency than prior art circuits by avoiding unnecessary energy losses through precision charging of the energy storage element in shunt with the flashtube.
A further object is to provide a strobe light circuit utilizing lower peak charging currents in order to minimize surges and possible tripping of circuit breakers or blowing of fuses.
A still further object is to provide a strobe light circuit that can be driven by either a D.C. voltage input or a full wave rectified voltage input.